1. Field of the Invention
This invention relates to a process and a device for measuring the state of charge of an electrochemical generator during charging or discharging of the generator. The invention applies mainly to lead generators, but it can also be applied to nickel generators or others.
2. Description of the Prior Art
Such devices as above noted are known which are based on several different principles:
ampere-hour meter devices which integrate the amperage of the generator and make it possible to count the ampere-hours delivered during an active and;
voltammetric devices, which by reference to a discharge curve use the voltage directly from the generator to describe the state of charge of the generator.
Voltammetric devices deduce the no-load electromotive force from the measurement of the voltage and of the current delivered by the generator through the a priori knowledge of the internal resistance of the generator and Ohm's Law.
These devices of the prior art exhibit numerous drawbacks such as:
poor knowledge of the initial capacity of the battery, misreading of the exact efficiency during the recharge, ignoring of the self-discharge;
a noncontinuous operation and ignoring low discharges;
presence of shunts to be calculated according to the applications and ignorance of the variation of the internal resistance of the generator while operating;
in the majority of the application made so far, the lack of operation during recharging of the generator.
Moreover, there is known, by French patent application FR No. 2,473,727, a process and an apparatus for determining the capacity of a battery according to which the no-load voltage V.sub.M of the battery during its initial connection, which represents its electromotive force E (to) at time to of the initial connection, is measured, then the charge or discharge voltage Vb of the battery at time t.sub.1 is measured, the difference .DELTA.V=E(to)-Vb is calculated, and the calculation is made of the value of the electromotive force E(t.sub.1) of the battery representing its state of charge at time t.sub.1 as a function of the difference .DELTA.V and its electromotive force at time to.
Unfortunately, this patent application exhibits a basic flaw which makes it unusable for a specialist of the art to the extent that it does not indicate how the electromotive force during charge or discharge, at time t.sub.2, is calculated as a function of .DELTA.V and of the electromotive force E (to) at the time of initial connection to.
It is known that the terminal voltage of a battery during charge or discharge depends at the same time on its state of charge and its speed or rate of charge or discharge. This is clear from the family of curves in FIG. 1A which illustrate the relation between voltage V of each cell of a lead storage battery expressed in volts and its period of discharge d expressed in hours. On this family, curve C.sub.1 represents a state of charge of the battery equal to 50% of its capacity, curve C.sub.2 a state of charge equal to 25% of its capacity and curve C.sub.3 the final voltage of the cell for various times or discharge rates. Thus, for example, a voltage of 1.90 volts measured on a cell will correspond to a residual capacity of 50% with a discharge rate of two hours, and to residual capacity of about 25% with a discharge rate of 8 hours.
It is clear from the above that it is not possible, in the current state of knowledge about the functioning of electrochemical generators, to deduce the residual capacity of a battery during charge or discharge from the sole knowledge of its terminal voltage. Actually, it is further necessary to know its discharge rate. Now, this parameter is particularly difficult to measure since the batteries are generally subjected to a discontinuous operation, particularly in automobiles where the current delivered can vary in significant proportions over short periods.